This invention relates to a carbon dioxide or steam drive process for producing a viscous oil or tar. More particularly, the invention provides a process for recovering oil from a subterranean viscous oil reservoir, or at least a portion of such reservoir, in which the stratification due to differences in permeabilities of the individual areas of rocks is not sufficient to cause a significant portion of the injected gas to flow through only one or a few layers of exceptionally high permeability.